We have previously purified the major surface glycoprotein (MSG) of both rat and human pneumocystis using high-performance liquid. Subsequently, we identified a number of clones from a cDNA library of P. carinii that contain genes encoding for the MSG. These clones are clearly related but not identical, demonstrating that multiple genes encode the MSG. Over the past year, we have continued studies to characterize the genetic organization and regulation of this family of genes. We have found that these genes are organized as tandem repeats, and that a single or limited number of sites regulate expression of the genes. Our collaborators at UCSF have demonstrated that a single upstream conserved sequence encoding a leader peptide is potentially expressed with each variable downstream MSG sequence. We have cloned a number of human P. carinii MSG genes and are currently attempting to produce recombinant protein. Expressing some of these genes should allow studies of immune responses to P. carinii in humans. We are also studying the region upstream of the expression site, which presumably plays a role in MSG expression, for potential promoter activity. In addition, we have begun studies to characterize other P. carinii antigens. The goal of this study is to better understand the pathogenesis of P. carinii pneumonia with the hope that we can use this information to control or prevent this disease. P. carinii is a major opportunistic pathogen of immunocompromised patients. Because P. carinii cannot be cultured, molecular approaches have been used to identify and characterize antigens of this organism. Recombinant antigens can then be used to examine host immune responses to P. carinii infection. We have an ongoing project to characterize the antigens of both rat and human P. carinii.